1. Field of the Invention
The present invention relates to a method for producing carbon nanotubes, in particular, it relates to a technique for producing single-walled carbon nanotubes.
2. Description of Related Art
Recently, carbon nanotubes have given rise to expectations not only as materials having a superior mechanical strength, but also as materials having field emission functions, hydrogen absorption functions, and magnetic properties. Carbon nanotubes of this kind are also called xe2x80x9cgraphite whiskersxe2x80x9d as referred to in Japanese Unexamined Patent Application, First Publication, No. Hei 5-146592; xe2x80x9cfilament carbonsxe2x80x9d or xe2x80x9cgraphite fibersxe2x80x9d as referred to in Japanese Unexamined Patent Application, First Publication, No. Sho 61-136992; xe2x80x9cextra fine carbon tubesxe2x80x9d as referred to in Japanese Unexamined Patent Application, First Publication, No. Hei 6-345413; xe2x80x9ccarbon tubesxe2x80x9d as referred to in Japanese Unexamined Patent Application, First Publication, No. 8-121207; xe2x80x9ccarbon fibrilsxe2x80x9d as referred to in Japanese Unexamined Patent Application, First Publication, No. 2-503334; and xe2x80x9ccarbon micro tubesxe2x80x9d or xe2x80x9ccarbon nanofibersxe2x80x9d as referred to in the journal xe2x80x9cCARBONxe2x80x9d (Vol. 174, page 215, 1996).
Although various methods for producing carbon nanotubes have been proposed, as explained in Japanese Unexamined Patent Application, First Publication, No. Hei 06-280116, most of these methods produce only multiwalled carbon nanotubes. A multiwalled carbon nanotube is a carbon fiber consisting of a plurality of graphen sheets.
However, in order to effectively utilize the above-described field emission functions, hydrogen absorption functions, or magnetic functions, it is important to simplify the structure of the carbon nanotubes. Therefore, it is necessary to produce, by an industrial method, a large amount of single-walled carbon nanotubes respectively consisting of single-layered graphen sheets.
Single-walled carbon nanotubes are also called xe2x80x9cfullerene pipesxe2x80x9d as referred to in Science, 1998, vol. 280, page 1254, or xe2x80x9ccarbon single tubesxe2x80x9d as referred to in Japanese Unexamined Patent Application, First Publication, No. Hei 8-91816.
In the conventional method, carbon nanotubes are produced, for example, by vapor deposition in which an arc discharge is generated using carbon electrodes and the carbon vapor is deposited on the surface of one electrode. In this method, it is also known that single-walled carbon nanotubes can be selectively produced by using metal catalysts and by controlling the pressure, the composition of the atmospheric gas, and the composition of the raw materials.
For example, Japanese Unexamined Patent Application, First Publication, No. Hei 7-197325 and Japanese Unexamined Patent Application, First Publication, No. Hei 9-188509 respectively disclose methods for producing single-walled carbon nanotubes using, as a metal catalyst, iron, cobalt, or nickel. Also, in Science, 1996, vol. 273, page 483, vapor deposition of single-walled carbon nanotubes by laser abrasion using metal catalysts such as cobalt and nickel is reported.
However, in the above methods, single-walled carbon nanotubes having straight shapes cannot be produced, instead, only those having helical shapes are produced. Therefore, these methods have the drawback that the application of the produced single-walled carbon nanotubes is narrowly limited to a particular field of the art. Furthermore, although it is expected that single-walled carbon nanotubes will provide superior properties when the diameter distribution thereof is narrow, because the diameter distribution of the single-walled carbon nanotubes produced by the prior methods is broad, there are the drawbacks that the function of the single-walled carbon nanotubes is not consistent.
It is an object of the present invention to produce single-walled carbon nanotubes having a straight shape at a high production efficiency. Also, it is another object of the present invention to produce single-walled carbon nanotubes having both small diameters and a narrow diameter distribution.
In order to accomplish the above objects, the method of the present invention comprises the step of developing carbon nanotubes by contacting carbon vapor with a non-magnetic transition metal including at least two elements selected from the group consisting of ruthenium, rhodium, palladium, and platinum.
The carbon vapor and particulates of a non-magnetic transition metal may be generated by arc discharge. The arc discharge may be performed in a reactor chamber containing an inert gas or the mixture of an inert gas and hydrogen gas, at a pressure ranging from 50 to 1500 Torr.
The arc discharge may be performed between a tip of a rod-shaped anode which contains carbon and the non-magnetic transition metal, and a tip of a rod-shaped cathode arranged so as to direct the tip toward the tip of the anode. The carbon vapor and the particulates of a non-magnetic transition metal will thereby be generated, and carbon nanotubes will be deposited on the circumferential surface of the base portion of the cathode.
The non-magnetic transition metal is preferably one selected from the group consisting of a binary mixture of ruthenium and rhodium, a binary mixture of ruthenium and palladium, a binary mixture of ruthenium and platinum, a binary mixture of rhodium and palladium, a binary mixture of rhodium and platinum, and a binary mixture of palladium and platinum.
The electrode used for generating arc discharge in order to produce carbon nanotubes contains at least two elements selected from the group consisting of ruthenium, rhodium, palladium, and platinum, and in particular, it is preferably the above mentioned binary mixture. The weight ratio of the non-magnetic transition metal to the graphite powder (metal:C) may be in the range of 2:1-10:1.
The composite which can be obtained by the method of the present invention includes particulates of non-magnetic transition metal and single-walled carbon nanotubes straightly extending from the surfaces of the particulates. The average grain size of the particulates of the non-magnetic transtion metal may be 10-20 nm, and the length of the single-walled carbon nanotubes extending from the surfaces of the particulates may be 1-1.28 nm.
According to the present invention, by using non-magnetic transition metal as a catalyst for vapor deposition, it is possible to produce, at a high yield, single-walled carbon nanotubes which have straightly extending shapes, not spirally wound shapes. Also, the single-walled carbon nanotubes obtained by the present method have small diameters, a narrow diameter distribution, and superior properties. Furthermore, the control of the average diameter and the diameter distribution of single-walled carbon nanotubes can be easily performed by controlling the pressure in the reactor chamber.
Additionally, because the non-magnetic transition metal does not have ferromagnetism, the magnetic property of the single-walled carbon nanotubes are not inhibited by the catalyst, and the characteristics of the products are not affected. Therefore, the products of the method of the present invention can be widely used for various kind of applications.